The invention relates to a fluoroscopic computed tomography method, to a computed tomography apparatus for carrying out such a method, and to a computer program for the processing of the measuring values produced by such a tomography apparatus.
EP 948930 describes a fluoroscopy system which can at the same time be used as a CT system (CT=computed tomography) for forming a three-dimensional CT image (volume CT). The fluoroscopy system comprises a gantry with a radiation source and a flat detector which rotate once about an axis of rotation within a time interval of between 5 and 90 seconds, thus producing from 300 to 500 two-dimensional fluoroscopic images wherefrom a three-dimensional CT image can be reconstructed.
However, in the context of the present invention fluoroscopic computed tomography is to be understood as a method which forms not only a single three-dimensional CT image, but also continuous CT images which are three-dimensional representations at successive imaging instants of the object to be examined, the distance in time between two successive instants being smaller than the period of time required for a rotation of the object to be examined (in the case of industrial objects) or the radiation source (in the case of medical examinations) about the axis of rotation.
A method of this kind is required, for example, for a CT-guided biopsy where a biopsy needle is introduced into an object to be examined and the progress of this biopsy needle is continuously checked on the basis of a series of three-dimensional CT images. For such examinations it is important that the CT images represent an as wide as possible zone of the object to be examined and that they contain as few motional artefacts as possible. The motional artefacts are caused by the fact that the reconstruction utilizes measuring values acquired at different instants.
Therefore, it is an object of the invention to provide a computed tomography method which meets the described requirements. This object is achieved in accordance with the invention by means of a fluoroscopic computed tomography method which includes the steps of:
continuously irradiating an object to be examined by means of a conical radiation beam which is emitted by a radiation source while the object to be examined or the radiation source rotates about an axis of rotation,
continuously acquiring, while using a detector unit, measuring values which are dependent on the intensity in the radiation beam beyond the object to be examined,
continuously reconstructing CT images which represent the object to be examined at imaging instants which are situated at a distance in time from one another which is shorter than the period of time required for a rotation of the object to be examined or the radiation source about the axis of rotation, the reconstruction including the following steps:
a) rebinning the measuring values so as to form a number of groups, each group containing the measuring values of fan beams which are formed during the irradiation and are situated in planes extending parallel to one another and to the axis of rotation,
b) one-dimensional filtering of the data of each group as produced by the rebinning in order to produce filtered data,
c) backprojecting the filtered data of a plurality of groups, for each voxel to be reconstructed there being used filtered data from the last fan beams formed prior to the relevant imaging instant, said fan beams having irradiated the relevant voxel from an overall projection angle range of 180xc2x0.
A computed tomography method which includes the steps a) and b) is known from an article by Grass et al in Phys. Med. Biol. 45(2) 329-347 or from the EP-A 990892. In the cited publications it is explained that there is a central, cylindrical zone which has flat side faces extending perpendicularly to the axis of rotation and within which each voxel is irradiated by the radiation source from an angular range of at least 180xc2x0. Reconstruction of the attenuation of the radiation cannot be performed with adequate quality outside this zone. The central zone, therefore, is the largest possible zone with flat side faces for which a CT image can be reconstructed. This aspect is used for the invention.
The cited publications elucidate that the central zone contains a first sub-volume which extends perpendicularly to the axis of rotation and is conically tapered towards the sides, the voxels present therein being exposed to radiation in all positions of the radiation source, and also contains a second sub-volume whose voxels are not exposed to the radiation in all radiation source positions. According to the known method, filtered data of fan beams which have irradiated the relevant voxel from an overall projection angle range of 360xc2x0 are used for each voxel to be reconstructed in the first sub-volume. However, in accordance with the invention for each voxel only filtered data of fan beams which have most recently irradiated the relevant voxel from an overall projection angle range of 180xc2x0 are used for the two sub-zones of the central zone. The period of time required for the acquisition of the fan beams used for the reconstruction is then shorter than in the known method, so that the artefacts caused by motion during the acquisition of the fan beams are less pronounced.
As has already been stated, the zone which can be reconstructed contains a sub-volume whose voxels are not continuously exposed to a fan beam. A part of the voxels in this sub-volume is exposed to all fan beams in an angular range of 180xc2x0 as the last fan beams prior to the imaging instant. The reconstruction for these voxels is analogous to the reconstruction for the voxels continuously exposed to fan beams. The other voxels of this second sub-volume are exposed to all fan beams which have been acquired in an angular range of 180xc2x0 as the last fan beams prior to the imaging instant. Therefore, in order to reconstruct these voxels it is necessary to use fan beams which are situated further back so that an overall projection angle range of 180xc2x0 is obtained for the fan beams used for the reconstruction of these voxels.
In these cases there may be groups of fan beams which are situated diametrically opposite one another. The invention describes alternative solutions for the processing of the filtered data of these fan beams. The solution in conformity with one aspect of the invention offers a potentially better signal-to-noise ratio because the filtered data of both fan beams is used, whereas the solution in conformity with another aspect potentially involves less motional artefacts because the filtered data of the fan beam acquired earlier is not taken into account.
In accordance with another aspect of the present invention a computed tomography apparatus is provided for carrying out the method of the present invention. In accordance with another aspect of the present invention, a computer program is provided for the processing of the measuring values of a computed tomography apparatus as set forth in accordance with the present invention.
In accordance with one aspect of the invention, a fluoroscopic computed tomography method is provided. The method includes the steps of continuously irradiating an object to be examined by means of a conical radiation beam which is emitted by a radiation source while the object to be examined or the radiation source rotates about an axis of rotation, continuously acquiring, while using a detector unit, measuring values which are dependent on the intensity in the radiation beam beyond the object to be examined, continuously reconstructing CT images which represent the object to be examined at imaging instants which are situated at a distance in time from one another which is shorter than the period of time required for a rotation of the object to be examined or the radiation source about the axis of rotation. The reconstruction includes the following steps: rebinning the measuring values so as to form a number of groups, each group containing the measuring values of fan beams which are formed during the irradiation and are situated in planes extending parallel to one another and to the axis of rotation; one-dimensional filtering of the data of each group as produced by the rebinning in order to produce filtered data; backprojecting the filtered data of a plurality of groups, for each voxel to be reconstructed there being used filtered data from the last fan beams formed prior to the relevant imaging instant, said fan beams having irradiated the relevant voxel from an overall projection angle range of 180xc2x0.
In accordance with a more limited aspect of the invention, the filtered data of fan beams which have irradiated the relevant voxel from projection angle ranges (xcfx86A-xcfx86B and xcfx86D-xcfx86E) which have been shifted 180xc2x0 relative to one another and are situated within the overall projection angle range of 180xc2x0 are summed in a weighted fashion in order to reconstruct the absorption of voxels which are only temporarily present in the fan beam.
In accordance with a more limited aspect of the invention, the reconstruction of the absorption in voxels which are only temporarily present in the fan beam utilizes, from among the filtered data of the fan beams which have irradiated the voxel from directions which are shifted 180xc2x0 relative to one another and are situated within the overall projection angle range of 180xc2x0, only the data (xcfx86A-xcfx86B) of the last fan beam formed.
In accordance with another aspect of the invention, a computed tomography apparatus for carrying out the method of the invention is provided. The apparatus includes: a radiation source (S) for continuously generating a conical radiation beam; a drive device for realizing a circular relative motion between the radiation source and the object to be examined; a detector unit for the continuous acquisition of measuring values which are dependent on the attenuation of the rays in the examination zone; and a reconstruction unit for the continuous reconstruction of CT images which represent the object to be examined at imaging instants whose distance in time from one another is shorter than the period of time required for a rotation of the object to be examined or the radiation source about the axis of rotation. The reconstruction unit processes the measuring values as follows: rebinning the measuring values so as to form a number of groups, each group containing the measuring values of fan beams which are formed during the irradiation and are situated in planes extending parallel to one another and to the axis of rotation; one-dimensional filtering of the data of each group as produced by the rebinning in order to produce filtered data; backprojecting the filtered data of a plurality of groups, for each voxel to be reconstructed there being used filtered data from the last fan beams formed prior to the relevant imaging instant, said fan beams having irradiated the relevant voxel from an overall projection angle range of 180xc2x0.
In accordance with another aspect of the invention, a computer program for processing the measuring values of a computed tomography apparatus of the present invention is provided. The program includes the steps of: rebinning the measuring values so as to form a number of groups, each group containing the measuring values of fan beams which are formed during the irradiation and are situated in planes extending parallel to one another and to the axis of rotation; one-dimensional filtering of the data of each group as produced by the rebinning in order to produce filtered data; backprojecting the filtered data of a plurality of groups, for each voxel to be reconstructed there being used filtered data from the last fan beams formed prior to the relevant imaging instant, said fan beams having irradiated the relevant voxel from an overall projection angle range of 180xc2x0.